Robert Webb’s first book, the memoir How Not to Be a Boy, established that as well as being funny on the telly he could write both sensitively and well. His first novel confirms it: it’s well paced, nicely written and highly entertaining. It’s also a very rum concoction indeed – as if someone had sandwiched a David Nicholls novel in the middle of a comedy thriller, using a Tardis.

When we first meet Webb’s protagonist Kate Marsden, she’s in her mid-40s, drunk, unwashed and suicidal. She is mourning the loss of her husband Luke, who died suddenly as a result of an undiagnosed brain tumour. Luke was an aspiring novelist who had been working on the same awful manuscript for 28 years. The awfulness hadn’t changed but every few years the title had: “From Whatever to Whenever to Why? to, briefly, Who Cares? And then triumphantly back to Whatever.” But with a Houllebecq novel of the same title, “Luke worried that he would be accused of plagiarism and that was not to be borne. By the time of his death, Luke’s unpublished masterpiece was called Fuck Off.”

Nevertheless, the reader is persuaded to credit Kate’s sadness that he’s gone. Indeed, the badness of his book may – as she now thinks with an inner howl of self-reproach – have been a missed clue that all was not right in his cranium. She’s learned that this tumour was growing, silently, even when they first met and fell in love at their university freshers’ week. If only … if only … But there’s nothing to be done now.

Her plan is to quit her job (she works as an IT expert for a PR consultancy run by a smarmy and amoral former university friend), kick over the traces (she has come into possession of some devastating kompromat on an unsavoury Russian client of the company, and threatens to go public with it), and then kill herself. Her grief is feelingly evoked, and just about jostles along tonally with the cartoony interactions with her horrible boss and the hectic implausibility of the plot elements.

The reader straps in, not really crediting the suicide plan, to expect a more or less cheerful caper including car chases, Russian heavies pursuing stolen USB drives, hairs-breadth escapes and suchlike. But then something odd happens. Kate falls asleep and wakes up in her university bed in 1992, in her 18-year-old body. She’s just hours away from her first meeting with her future husband. And this time she’s determined to do it right. She has a second chance to save him.

We’re not in a comedy thriller, now, but in a wistful science-fictional romance of the Post-Birthday World/Time-Traveler’s Wife type – about regrets and fate and falling in love and quantum possibilities and so on. Webb ladles on the period detail with gusto: Kate is constantly reaching for her absent mobile phone, forgetting when rail privatisation happened, or blurting out “European Union” rather than “European Commission”. She’s also up to her ears in leaded petrol, Ned’s Atomic Dustbin T-shirts and people smoking indoors.

Again, it’s funny and well done, and in parts very poignant: not least because the author has thought through the way in which the perspective of the older Kate will change her experience of her younger self. The boy she met as a 19-year-old turns out to be pretentious and annoying: falling in love with him might not be all that easy. “Luke really was a stranger, and the man was yet to grow around him. And not just grow – the mature Luke would take work to build and Kate had a pretty good idea that it was she who had laid about half the bricks.”

What if you had your time again, knowing what you know now and with your feelings and memories intact? Would you hug your family closer? Would you warn your new-old university pals off the people who you know are going to be bad for them? Would you even fall in love with the same person? Webb makes a creditable but not completely successful attempt at weaving these giant philosophical questions through a do-you-remember-the-90s student social comedy.

It wouldn’t be proper to give spoilers – but I think I can reveal that the present-day comic caper plot in due course comes back with bells on: something more in the direction of Kingsman than James Bond. Did I mention Kate is also a former international silver medallist at karate? That’s relevant too. The actual science-fiction mechanism by which all this works is not just left unexplained, but actively propelled into massive paradox territory in the closing pages.

As I say, a rum concoction. In fact, it’s exactly the sort of rum concoction you’d have downed for a bet in freshers’ week in 1992. You’d have the night of your life, wake up oddly weepy, and wonder what the hell all that was about.

• Come Again is published by Canongate (RRP £16.99).